1. Field of the Invention
The invention relates to a transmission unit, in particular a multirange transmission.
2. Description of the Related Art
Transmission systems in the form of power distribution transmissions embodied as superposition gears are known in numerous designs. Reference is made to the following documents by way of example:
1. U.S. Pat. No. 6,921,349 B2
2. DE 197 55 612 A1
3. EP 1061 287 A2
4. DE 43 08 761 A1
5. DE 887 457 C
The design according to DE 197 55 612 A1 includes a transmission input shaft and a continuously variable step-up gear coupled to the transmission output shaft in the form of a flexible drive having an input and an output, the input being connected in a rotationally fixed manner to the transmission input shaft, a fixed gear ratio stage, and a superposition gear having a first input stage which is connected in a rotationally fixed manner to the output of the continuously variable step-up gear. In addition, a second input stage is provided which by way of a first clutch may optionally be connected to the transmission input shaft via the fixed gear ratio stage, in addition to an output stage which is coupled to the transmission output shaft in a rotationally fixed manner. On the drive side the fixed gear ratio stage is coupled in a rotationally fixed manner to the transmission input shaft, and with respect to the fixed gear ratio stage on the output side the first clutch is positioned in such a way that it optionally connects the second input stage of the superposition gear on the output side to the fixed step-up gear. A functionally reliable multirange transmission may be easily provided by way of this approach. This approach offers the advantage that in a multirange transmission produced by a combination of a continuously variable step-up gear and a geared-neutral range, high meshing speeds in the region of the first clutch may be avoided, since as a result of the fixed gear ratio stage the clutch is provided at a location after a corresponding transformation of the high rotational speed of the drive shaft to the low rotational speed. This reduces wear and increases the service life of the first clutch. However, a significant disadvantage lies in the direct connection between the continuously variable transmission, also referred to as CVT, and the transmission input and, therefore, the drive shaft. The continuously variable transmission is thus always coupled to the rotational speed of the drive motor. Load is reduced on the superposition gear in a region of high rotational speeds of the drive shaft, i.e., lower gear ratios of the continuously variable step-up gear, by providing a second clutch which optionally connects the first input stage to the output stage of the superposition gear. This establishes a rigid connection between the output shaft of the continuously variable step-up gear and the output shaft, thereby bridging the superposition gear in the torque flow. Another significant problem of power transmission via the continuously variable step-up gear is that, as a result of its dimensions, the step-up gear is able to transmit only a maximum allowable torque, since otherwise impermissible slip conditions would be observed at very high loads which would result in increased wear on the traction way. Because of the direct coupling of the continuously variable transmission to the transmission input, however, the latter is continuously exposed to these conditions. In other words, the input of the CVT is impinged on by the rotational speed at the transmission input, and therefore, by the drive motor.
A transmission unit is known from U.S. Pat. No. 6,921,349 B2 which has a structure that has been modified such that the load on the flexible drive transmission is significantly reduced, thus ensuring high power, in particular higher power than in a design according to DE 197 55 612 A1, to be transmitted via this flexible drive transmission. In this design the transmission unit is likewise embodied as a superposition gear unit. This superposition gear includes a transmission input and a transmission output, in addition to two superposition gears located between the transmission input and transmission output and connected to one another. Each of the two superposition gears is designed as a three-shaft planetary gear. Both are interconnected to form a four-shaft planetary gear. A continuously variable transmission in the form of a flexible drive transmission is connected between the first superposition gear and the second superposition gear. Each planetary gear includes a sun wheel, an internal gear, planet wheels, and a bridge. The individual shafts are formed by the sun wheel, internal gear, or bridge of the respective superposition gear. The transmission input is connected in a rotationally fixed manner to a first shaft of the first superposition gear and to a first shaft of the second superposition gear. The transmission output is connected in a rotationally fixed manner to a second shaft of the first superposition gear and to a second shaft of the second superposition gear. The coupling of the two three-shaft planetary gears to form a four-shaft planetary gear occurs by connection of the first and second shafts of the first and second superposition gears. The configuration of the continuously variable transmission in the form of a flexible drive transmission is achieved between the third shafts of the first and second superposition gears. The term “shaft” is understood in a functional sense, and includes either the individual elements of the planetary gear (sun wheel, internal gear, or bridge), or the elements connected thereto in a rotationally fixed manner, for example in the form of shafts or hollow shafts. Depending on the operating state, the individual shafts assume the function of inputs and outputs. Thus, for the transmission of power from the transmission input shaft to the transmission output shaft via the continuously variable transmission, the first superposition gear includes one input and two outputs. The input is formed by the first shaft, whereas the first output, which is at least indirectly connected to the continuously variable transmission, is formed by the third shaft, and the second output which is coupled in a rotationally fixed manner to the transmission output shaft is formed by the second shaft. In this operating state the second superposition gear includes one input and one output, the input likewise being coupled to the transmission input shaft and formed by the first shaft of the second superposition gear, and the output being formed by the second shaft. The third shaft is connected to the continuously variable transmission. In addition, a way is provided for changing the gear ratio at the transmission. One of the two superposition gears—the first or the second superposition gear—has pairs of intermeshing planetary gears between the sun wheel and the internal gear. The sun wheel and internal gear are rotatably supported on the bridge. The pairs of intermeshing planetary gears are also referred to as double-barreled planet wheels. On account of the design of the second superposition gear as a planetary gear having pairs of intermeshing planetary gears, also referred to as double-barreled planetary gears, for a portion of the overall operating range it is ensured that the CVT operates at maximum rotational speed, whereby a change may also be made with regard to the gear ratio at the individual disks at maximum engine speed; i.e., rotation above the zero level is possible, thus enabling a geared-neutral state as well as a change in rotational direction to be achieved by the transmission according to the invention. The double-barreled design offers the advantage that for an increased rotational speed thus produced at the output coupled to the continuously variable transmission, in particular the internal gear of this planetary gear, a reduction at the output of the other respective planetary gear, in particular the internal gear, which is coupled to the continuously variable transmission is achieved corresponding to the design of the other planetary gear. According to this design, however, it is not possible to allow the continuously variable transmission to operate multiple times over the entire operating range at maximum rotational speed.
A multirange transmission is known from EP 1 061 287 A2. This multirange transmission is characterized by a three-shaft planetary gear which may be connected to the transmission output via a continuously variable transmission. Three-shaft planetary gears are always traversed in parallel, the coupling to the transmission output being achieved by way of different gear ratios, which in this case are implemented via spur gear stages which may be respectively coupled to the transmission output via individual clutch units. In other words, only one gear ratio stage, which is fixed, is provided downstream from the continuously variable transmission. As a result, however, the continuously variable step-up gear is consistently varied via a fixed gear ratio. Depending on the output gear ratio selected, this results in individual maximum allowable gear ratio ranges. This also applies analogously to the design described in DE 887 457 and DE 43 08 761.
What is needed in the art is a multirange transmission which, in addition to the advantages achieved by the transmission configurations previously described, provides an improved transmission configuration, whereby in particular operation above the zero level is possible. What is also needed is to reduce the load on the flexible drive transmission during operation. What is further needed is a design having the improved transmission configuration, described on the basis of U.S. Pat. No. 6,921,349 B2, which overcomes the referenced disadvantages from the prior art.